Lithium ion secondary batteries, which have small size and large capacity, have been widely used as power supplies for electronic devices such as mobile phones and notebook computers and have contributed to enhancing convenience of mobile IT devices. Attention is now drawn to use of such batteries in large-scale applications, for example, power sources for automobiles and the like, and power storage devices for smart grid.
It is urgent to further increase the energy density of lithium ion secondary batteries, and examples of the process to increase the energy density include a process of using active materials having large capacity, a process of increasing the operating potentials of batteries, and a process of enhancing the charge/discharge efficiency, cycle life and the like. Among these, the process of increasing the operating potentials of the battery is a measure effective for size and weight reduction of battery modules used in electric vehicles and the like because the process can provide fewer numbers of battery packs in series.
However, use of a high potential positive electrode, for example, causes problems due to decomposition of electrolyte solutions, such as gas evolution and a drop in capacity in long-term cycles and under high-temperature storage because the electrolyte solution is exposed to the high potential positive electrode for a long period. This problem becomes serious when using the positive electrode material having high charge and discharge potential especially at 4.5 V or higher. To solve these problems, it has been proposed to use various electrolytes.
Patent Document 1 discloses that the use of a compound having S—O bonds such as sulfolane or dimethyl sulfone in an electrolyte solution suppresses the decomposition of the electrolyte solution in a charged state, and suppress the increase in internal resistance. Patent Documents 2 to 4 disclose examples using an electrolyte solution containing a cyclic carbonate and an open chain sulfone. It discloses that the use of the electrolyte solution containing an open chain sulfone improves cycle characteristics at high temperature and storage characteristics, compared with an electrolyte solution based on only carbonates. Also, an electrolyte solution containing a compound other than a carbonate-based solvent and an open chain sulfone compound has been disclosed. Patent Document 5 discloses a non-aqueous electrolyte battery comprising an electrolyte containing an open chain sulfone compound and an open chain ester compound. Patent Document 6 discloses a non-aqueous solvent for an electricity storage device containing a fluorinated cyclohexane, an open chain sulfone compound and a cyclic sulfone compound.